Brief Answer:
Lack of good intestinal bacteria or too fast stool passage.

Detailed Answer:
Hello!

Thank you for the query.

The stool on its way from small intestine to rectum changes its color. At the beginning it is yellow. This is caused by a bile. Bile is secreted from the liver to duodenum. Thanks to intestinal bacteria, yellow bile is slowly digested to stercobilinogen. Stercobilinogen is brown. If the whole bile is digested, the stool is brown. If only half of bile is digested, we get a mixture of yellow and brown which is green.
So answering your question, lack of intestinal bacteria or too fast passage of stool are the most common reasons of green stool.
Please try to add some probiotic to your diet along with fiber. This should normalize the stool color. If not, please have stool test done.

Hope this will help. Feel free to ask further questions.
Regards.

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